JavaScript Strings

Quick Reference

Goal	Methods
Change the ends	trim(), slice(), padStart()
Check contents	includes(), startsWith()
Change characters	replace(), toLowerCase()
Break it apart	split()
Put it together	join(), +, template literals

The “Cleaners” (Formatting)

Used mostly for normalizing user input or preparing data for a database.

• trim() - Removes whitespace from both ends
• toLowerCase() / toUpperCase() - Essential for case-insensitive comparisons
  • Example: if (email.toLowerCase() === savedEmail)
• padStart(targetLength, padString) - Useful for formatting
  • Example: "5".padStart(2, "0") → "05"
• repeat(count) - Returns a string repeated n times

The “Analyzers” (Searching)

Used to check if a string contains what you need. All return Booleans or Indices.

• includes(substring) - Returns true if the string contains the substring
• startsWith() / endsWith() - Much faster than regex or slice
• indexOf(substring) - Returns the starting index, or -1 if not found

The “Extractors” (Substrings)

Used to pull specific parts out of a larger string.

• slice(start, end) - The modern gold standard
  • Pro-tip: Use negative numbers to count from the back: str.slice(-3) gets the last 3 characters
• split(separator) - Turns a string into an array
  • Example: "a,b,c".split(",") → ["a", "b", "c"]
• at(index) - Modern alternative to str[i], supports negative indices
  • Example: str.at(-1) gets the last character

The “Transformers” (Modifying)

Used to swap values or change the structure.

• replace(pattern, replacement) - Replaces the first match
• replaceAll(pattern, replacement) - Replaces all matches (ES2021)
• concat(...strings) - Joins strings together (most prefer + or template literals)

Essential Array Methods

Since you often convert between strings and arrays:

• join(separator) - The inverse of split(). Turns an array back into a string
• filter(callback) - Removes items that don’t match a condition
• map(callback) - Transforms every item in an array
• reduce(callback, initial) - Boils down an array into a single value